Micro lenses for a CCD are formed, e.g., by performing the steps of: forming a mask layer 11, out of a resist film, to have specific lens shapes on a lens material layer 10, which is made of a lens material such as an Si3N4 film; and then etching both the mask layer 11 and the lens material layer 10 to transfer the lens shapes of the mask layer 11 to the lens material layer 10, as shown in FIG. 24. Conventionally, in the etching step, the lens material layer 10 and the mask layer 11 are etched in a selectivity of 1:1, whereby the lens shapes of the mask layer 11 are precisely transferred to the lens material layer 10.
Normally, the shape of a micro lens is defined by the curvature, the height, and the width thereof wherein the width is a diameter of the lens from a horizontal viewpoint. Further, it is possible to form a desired lens shape by controlling various process parameters. Meanwhile, for the purpose of improving a sensitivity by increasing the light collection efficiency in terms of an uppermost surface lens of a CCD, it is required to enlarge an area of the lens, whereby more light can be collected on a condensing point. For this reason, it is preferred to enlarge a lens width by reducing a distance A between adjacent lenses.
However, the lens shapes of the mask layer 11 are formed in a photolithography process, which limits the ability to minimize of a distance B between adjacent lens shapes. That is to say, the minimum of the distance B is 0.2 to 0.3 μm and cannot be reduced further because adjacent resist films adhere to each other due to a surface tension thereof in the course of a heat treatment during the photolithography process.
Consequently, the present inventors have endeavoured to establish a method for forming micro lenses whose shapes are different from the lens shapes of the mask layer 11 by controlling a selectivity between the lens material layer 10 and the mask layer 11 during an etching process. In this connection, Japanese Patent Laid-Open Application No. 2000-164837 (hereinafter, referred to as “reference 1”) discloses a method for forming an in-layer lens with a thickness different from that of a lens shape of a resist layer by etching a lens material layer and the resist with the lens shape using an etching gas including SF6 gas and O2 gas, and controlling a ratio of the flow rates of the SF6 gas and the O2 gas; and Japanese Patent Laid-Open Application No. 2002-110952 (hereinafter, referred to as “reference 2”) discloses a method for forming on-chip lenses with large widths and narrowing a distance between adjacent lenses by etching a lens material layer and a resist film, where the lens shapes are formed, using an etching gas including CF4 and O2.
However, the method of reference 1 has a disadvantage in that the width of the resulting lens becomes rather small, because the etching gas includes O2. It is presumed that oxygen facilitates an etching of the resist film that is an organic film, whereby the lens shape of the resist becomes smaller and the reduced lens shape of the resist is transferred, to produce a smaller lens. For this reason, this method suffers from the lack of controllability over a lens area. Further, although this method can control the thickness of the lens, it cannot form a lens with a large width.
Further, the method of reference 2 has a problem in that an in-surface uniformity of an etching rate becomes lower in case the lens material layer is an inorganic film. Furthermore, the method of reference 2 has the same problem as that of reference 1 in that it is difficult to form a lens with a large width because the etching gas includes O2.